1. Field of the Invention
The present invention relates in general to the ultrasonic detection of hydrogen damage in boiler tubes, and in particular to a new and useful method and apparatus for confirming the presence of hydrogen damage, using movably mounted electromagnetic transducers (EMATs) which can be pressed closely against and at an accurate spacing or lift-off from the outer surface of a boiler tube having a range of different outside diameters (ODs).
2. Description of the Related Art
Starting in 1983, The Babcock & Wilcox Company developed a successful technique for the detection of hydrogen damage in fossil fired boilers. The technique was based on the attenuation of conventional ultrasonic waves in damaged sections of boiler tubes. Since that time, over 50 units have been inspected with excellent results. In many cases, a full inspection of the waterwall was requested. In one medium size unit, this required over 250 gallons of couplant and scanning over 10 miles of tubing and 1100 welds. In order to make this task easier, electromagnetic acoustic transducers (EMATs) were used to eliminate the need for couplant and increase the scanning speed. U.S. Pat. No. 4,685,334 discloses both conventional and EMAT techniques for detecting hydrogen damage in boiler tubes.
A new technique for hydrogen damage confirmation has been developed recently. A confirmation technique was needed since under certain conditions, such as extreme inside diameter (ID) pitting, both the conventional and EMAT techniques produced indications that could not be distinguished from actual hydrogen damage. The confirmation technique was based upon refraction of shear waves across a chord in a boiler tube. This technique was field tested using conventional ultrasonics in a unit which had extreme outer diameter (OD) erosion on a large section of the waterwall. It was clearly demonstrated that the chordal technique was superior under those adverse conditions. In addition a tube was removed, sectioned, and etched with nital in the field. Hydrogen damage was confirmed at the exact location indicated by the chordal technique.
There are some disadvantages to the chordal technique using conventional ultrasonics. A different beam angle must be used for each combination of tube OD and wall thickness. This requires a pair of special wedges for each particular tube-wall combination.
A comprehensive description of the structure and operation of EMATs, can be found in "Evaluating EMAT Designs for Selected Applications", B. W. Maxfield, et al Materials Evaluation, 45, October, 1987.
Additional information on non-destructive ultrasonic and electromagnetic acoustic devices and testing can be found in U.S. Pat. Nos. 4,127,035; 4,307,612; 4,320,661; and 4,593,568.